Optical recording media of a structure in which there are a plurality of recording layers and the recording and/or reproducing of information is/are performed for each of these plurality of recording layers are conventionally known. As shown in FIG. 10, in an optical recording medium 101 of such a structure, a first recording layer 103, an intermediate layer 104, a second recording layer 105 and a cover layer 106 are layered in this order on one principal surface of a substrate 102. In this optical recording medium 101, recording and/or reproducing is performed using laser light that is incident from the side opposite to the substrate 102, that is, from the side of the cover layer 106.
If the optical recording medium 101 shown in FIG. 10 is an optical recording medium of a phase change type, the first recording layer 103 and the second recording layer 105 include a phase change recording film which is reversibly changed between a crystalline state and an amorphous state by recording light, and in which these two states can be detected by the difference in reflectance for reproducing light.
In addition, in the first recording layer 103 and the second recording layer 105, a metal material film containing an Al alloy material or an Ag alloy material is formed adjacent to the phase change recording film. This metal material film has the function of enhancing, at the time of reproducing, signals recorded on the phase change recording film, and a heat sink function for rapidly cooling the phase change recording film at the time of recording.
Incidentally, in this optical recording medium 101, sufficient absorption or reflection when recording on and reproducing from the second recording layer 105 is demanded of the second recording layer 105 positioned on the side from which laser light enters, while at the same time it is necessary to transmit laser light of sufficient intensity for recording on and reproducing from the first recording layer 103 when recording on and reproducing from the first recording layer 103.
Since there exists in the phase change recording film and the Al alloys or Ag alloys described above absorption with respect to laser light, enhancing the transmittance of the second recording layer 105, in particular, is an important issue.
As a method of enhancing the transmittance of the second recording layer 105, reducing the thickness of the metal material film constituting the second recording layer 105 may be considered. However, when the thickness of the metal material film is made thinner than the present thickness, the reflectance of the second recording layer 105 may become lower, or its heat capacity as a heat sink may become lower. Therefore, it is difficult to adopt this method for making the thickness of the metal material film thinner.
In addition, as another method of enhancing the transmittance of the second recording layer 105, reducing the thickness of the recording film, such as the phase change recording film, constituting the second recording layer 105 may be considered. However, making the thickness of the recording film thinner leads to a reduction in the number of crystal nuclei in the recording film, and thus the transfer rate during recording may become lower.
As such, the present invention is one that is proposed in order to solve such conventional problems, and makes it its object to provide an optical recording medium including two or more recording layers, the optical recording medium being capable of transmitting at a high transmittance recording light or reproducing light for performing recording on and reproducing from the first recording layer without making the thickness of the second recording layer positioned on the side from which the recording light or reproducing light enters thinner.